Hematopoietically-derived cells, including cells such as neutrophils, monocytes, dendritic cells, eosinophils and lymphocytes, are important cellular mediators of the inflammatory response and respond to soluble inflammatory mediators by migration to the site of tissue injury or infection where the newly arrived cells perform their effector functions.
Neutrophils which represent 40-50% of the circulating leukocyte population are particularly important to both immunity and inflammation. Neutrophils are normally quiescent cells but upon stimulation can mediate a variety of different inflammatory activities. A large number of different agents are capable of activating neutrophils and this activation is normally mediated by binding of the activating agent to specific receptors expressed on the surface of neutrophils. Once activated, the neutrophils are capable of binding to endothelial cells and migrating to the site of tissue damage, a pathogen or a foreign material. Similarly, eosinophils are also potent inflammatory effector cells, although these cells are most often associated with allergic diseases such as asthma. Like neutrophils, eosinophils have a potent armory of proinflammatory molecules that can initiate and maintain inflammatory responses.
Once at the inflammatory site, recruited cells such as eosinophils and neutrophils induce further inflammation by releasing inflammatory products and recruiting other hematopoietically-derived cells to the site. In some cases, the inflammatory response mediated by the specifically recruited hematopoietically-derived cells protects the host from morbidity or mortality by eliminating the infectious agent. In other cases (i.e., autoimmunity, ischemia/reperfusion, transplantation, allergy), the inflammatory response further damages the tissue resulting in pathology. Thus, agents which alter inflammation or recruitment of cells may be useful in controlling pathology.
Although CD38 expression was at first believed to be restricted to cells of the B cell lineage, subsequent experiments by a number of groups have demonstrated that CD38 is widely expressed on both hematopoietic and non-hematopoietically-derived cells. Homologues of CD38 have also been found to be expressed in mammalian stromal cells (Bst-1) and in cells isolated from the invertebrate Aplysia californica (ADP-ribosyl cyclase enzyme) (Prasad GS, 1996, nature Structural Biol 3:957-964)
More recently, CD38 was shown to be a multifunctional ecto-enzyme with NAD+ glycohydrolase activity and ADP-ribosyl cyclase activity, enabling it to produce nicotinamide, ADP-ribose (ADPR), cyclic-ADPR (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) from its substrates NAD+ and NADP (Howard et al., 1993 Science 252:1056-1059; Lee et al., 1999 Biol. Chem. 380;785-793). Two of the metabolites produced by CD38, cADPR and NAADP, have been shown to induce the release of intracellular calcium in cells isolated from tissues of plants, invertebrates and mammals, suggesting that these metabolites may be global regulators of calcium responses (Lee et al., 1999 Biol. Chem. 380;785-793).
Both cADPR and NAADP are known to induce calcium release from calcium stores that are distinct from those controlled by Ip3 receptors (Clapper, D L et al., 1987, J. Biological Chem. 262:9561-9568). Instead, cADPR is believed to regulate calcium release from ryanodine receptor regulated stores, as agonists of ryanodine receptors sensitize cADPR mediated calcium release and antagonists of ryanodine receptors block cADPR dependent calcium release (Galione A et al., 1991, Science 253:143-146). Thus, it has been proposed that cADPR is likely to regulate calcium responses in tissues such as muscle and pancreas where ryanodine receptors are expressed. Interestingly, it was recently shown that the muscle fibers of the parasitic flatworm, S. mansoni, express ryanodine receptors and that agonists of ryanodine receptors such as caffeine can induce intracellular calcium release and muscle contraction in the parasite (Day et al., 2000 Parasitol 120:417-422; Silva et al., 1998, Biochem. Pharmacol 56:997-1003). In mammalian smooth muscle cells, the calcium release in response to acetylcholine can be blocked not only with ryanodine receptor antagonists, but also with specific antagonists of cADPR such as 8-NH2-cADPR or 8-Br-cADPR (Guse, A H, 1999, Cell. Signal. 11:309-316).
These findings, as well as others, indicate that ryanodine receptor agonists/antagonists including cADPR can regulate calcium responses in cells isolated from species as diverse as helminths to mammals, however, it is unclear whether ADP-ribosyl cyclase enzymes such as CD38 or SM38 are required for the production of cADPR in vivo. Additionally, there has been no direct evidence to link CD38 enzyme activity with downstream responses such as calcium release, proliferation, apoptosis, migration or other effector functions. Thus, despite the high level expression of CD38 on many cell types, no clear defining role for CD38 enzyme activity in immune responses has been established.